1. Field of the Invention
The present invention relates generally to electro-optical devices and, more particularly, to an exposure controller for use with an image pick-up device having an optical preamplifier such as an image intensifier coupled thereto.
2. Description of the Prior Art
In an image device such as an electronic camera, the product of light intensity, detector sensitivity, and time determines the amount of stored charge. Overexposure occurs when the product exceeds the charge storage capacity and information about the bright areas of the image is lost. Underexposure occurs when the product for the brightest areas is much less than the capacity and information about the darker areas of the image is lost. Exposure control of a camera seeks to adjust the intensity, sensitivity, and time product to optimize the image quality from the camera.
Camera exposure control can be performed either manually or automatically. To effect exposure control manually, the camera operator must adjust some component of the camera to achieve optimum image quality. For example, the operator may adjust the iris of the camera lens to admit more or less light. Manual exposure control, however, is often slow to respond to changes in scene illumination. Accordingly, automatic exposure control based upon the level of the video signal leaving the camera is often preferred because it can respond faster to such scene changes than manual operation and is simple to implement. However, even when automatic exposure control circuitry is present, if the change in illumination is too large from one field or frame to the next, saturation may obscure how much exposure adjustment is needed to accommodate the new light level, and many frames may be required to recover the correct exposure condition.
Specifically, when saturation occurs with a post storage or post readout exposure controller, the controller must search over several readout periods to re-establish the new, correct exposure level because the controller has no information about the degree of over exposure that occurred and must therefore follow a preprogrammed, all purpose recovery strategy. Furthermore, a second light sensor is generally needed if the exposure controller must get the correct exposure in a single frame.
Accordingly, it is an object of the present invention to provide an exposure controller circuit for a camera which is adapted to measure the intensity, sensitivity, and time product for each field or frame and which is operable in response to such measurements to adjust the exposure time in accordance with the charge storage capability of the pickup tube of the camera.
It is yet another object of the present invention to provide an exposure controller circuit which optimizes the signal to noise ratio of the camera pickup tube by adjusting the gain of an optical preamplifier coupled thereto.